Updating the #wp-auth-check login form to match the new admin design:

* Remove border-radius on the container and add a box shadow
* Style the top bar background to match the new #eee page background
* Replace the close button PNG with a Dashicon
* Remove a no-longer-needed button style override in buttons.css that caused padding to change when clicking the submit button.

See #25858.



git-svn-id: https://develop.svn.wordpress.org/trunk@26441 602fd350-edb4-49c9-b593-d223f7449a82